Orthogonal frequency division multiplexing (OFDM) and Orthogonal frequency division multiple access (OFDMA) systems divide an available bandwidth into a plurality of orthogonal frequency subcarriers. Various subsets of the subcarriers may be assigned for use in communications, such as communications between particular stations. The particular subcarriers and the number of subcarriers assigned for use with respect to a communication may be based upon such considerations as the bandwidth or throughput to be provided by the radio link, interference mitigation or avoidance, etcetera. In an OFDMA system, multiple stations (e.g., subscriber stations) may be simultaneously provided communication links with a common access point (e.g., base station) or other station by simultaneously assigning different subsets of the subcarriers for the links of the multiple stations.
In OFDM and OFDMA communications, a signal is split into a number of sub-signals which are then transmitted simultaneously on different ones of the subcarriers. These separate subsignals may then be recombined by a receiving station to form the original signal for further processing etcetera.
Communication access is typically provided to the various stations through a defined protocol, such as may require access, resource allocation, authorization, and registration. It is common to use a ranging process as part of an access protocol in OFDM and OFDMA systems. In a typical ranging process, a subscriber station desiring access to network resources transmits a ranging code on a pre-specified set of subcarriers. That is, the subscriber station transmits a ranging code spread over multiple subcarriers which form the ranging subchannel. The ranging code may be a random or quasi-random code (e.g., code division multiple access (CDMA) chip code). The base station extracts the ranging code from the received signal and estimates the corresponding time delay. The time delay is used by the base station for transmission time delay estimation used with respect to downlink and uplink resources assigned to the subscriber station for further communications.
In a multiple access system, other subscriber stations are generally transmitting data during times in which a subscribed station is transmitting a ranging request. Although interference from such simultaneous transmissions may be mitigated according to the cross-correlation property in an ideal environment, the channel is typically affected by multipath fading, etcetera. Such channel characteristics not only present interference such that a particular subscriber station's ranging code is more difficult to isolate in the received signal, but also present issues with respect to accurately determining the time delay associated with the ranging signal. That is, the first arriving path for the ranging signal provides the best estimate of time delay. However, the ranging processes implemented previously have been unable to reliably identify the ranging signal of the first arriving path. The following equations set forth prior art ranging signal processing for determining time delay in an OFDM system.
The received ranging signal may be represented as:r(n)=x(n)*h(n)  (1)Where, in the above equation, x(n) denotes the transmitted ranging signal and h(n) denotes the channel impulse response, represented as:
                              h          ⁡                      (            n            )                          =                              ∑                          i              =              0                                      L              -              1                                ⁢                                          ⁢                                    α              i                        ⁢                          δ              ⁡                              (                                  n                  -                                      d                    i                                                  )                                                                        (        2        )            To simplify the math operations, synchronization is performed in the frequency domain. Accordingly, the foregoing time domain equations may be transformed to the frequency domain using N-point fast Fourier transforms (FFT), such that:R(k)=X(k)H(k)  (3)Where, in the above equation:
                                          X            ⁡                          (              k              )                                =                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          x                ⁡                                  (                  n                  )                                            ⁢                              w                N                kn                                                    ⁢                                  ⁢                  and          ;                                    (        4        )                                                      H            ⁡                          (              k              )                                =                                                    ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                          ⁢                                                h                  ⁡                                      (                    n                    )                                                  ⁢                                  w                  N                  kn                                                      =                                          ∑                                  i                  =                  0                                                  L                  -                  1                                            ⁢                                                          ⁢                                                α                  i                                ⁢                                  w                  N                  kd                                                                    ,                            (        5        )            To determine a time delay of the received ranging signal, various different delays are tried and the partial channel estimation is computed by using inverse fast Fourier transform (IFFT) as below:
                                                                                             z                  ⁡                                      (                    m                    )                                                  =                                ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                                          ⁢                                                            R                      ⁡                                              (                        k                        )                                                              ⁢                    X                    *                                          (                      k                      )                                        ⁢                                          w                      n                                              -                        km                                                                                                                                                                ⁢                where                                                                                        =                                ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                                          ⁢                                                                                                                                      X                          ⁡                                                      (                            k                            )                                                                                                                      2                                        ⁢                                          H                      ⁡                                              (                        k                        )                                                              ⁢                                          w                      N                                              -                        km                                                                                                                                                                ⁢                                                                                                                                                          w                            N                                                    =                                                      exp                            ⁡                                                          (                                                                                                -                                  j2π                                                                /                                N                                                            )                                                                                                      ,                                                                                                                                                                          X                          ⁡                                                      (                            k                            )                                                                          =                                                  {                                                                                                                                                      ±                                  1                                                                                                                                                              k                                  ∈                                  p                                                                                                                                                                                    0                                                                                            otherwise                                                                                                                                                                                                                                                                                =                                ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                                          ⁢                                                            I                      ⁡                                              (                        k                        )                                                              ⁢                                          H                      ⁡                                              (                        k                        )                                                              ⁢                                          w                      M                                              -                        km                                                                                                                                                                ⁢                and                                                                                        =                                ⁢                                                      ∑                                          i                      =                      0                                                              L                      -                      1                                                        ⁢                                                                          ⁢                                                            ∑                                              k                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  α                        i                                            ⁢                                              I                        ⁡                                                  (                          k                          )                                                                    ⁢                                              w                        N                                                  -                                                      k                            ⁡                                                          (                                                              m                                -                                                                  d                                  i                                                                                            )                                                                                                                                                                                                                                              ⁢                                                      I                    ⁡                                          (                      k                      )                                                        =                                      {                                                                                            1                                                                                                      k                            ∈                            p                                                                                                                                                0                                                                          otherwise                                                                                                                                                                              (          6          )                    The peak IFFT value from the above channel equation is used to determine the time delay.
As shown above, prior ranging processes have merely relied upon peak detection with respect to the received ranging signal. However, the peak often does not correspond to the first path (i.e., the first path is often not the strongest path). Establishing time delay based upon the ranging signal as received in other than the first path results in improper timing and may cause undesired signal characteristics such as inter-symbol interference (ISI). That is, if the timing is determined by the maximum of the above IFFT and the first path is not the strongest path, part of the cyclic prefix of the next symbol will be included in the FFT window of the current symbol. Thus ISI will be generated due to wrong timing. Accordingly, the foregoing ranging signal time delay determination is less than optimal.
Further exacerbating the problems associated with ranging signal time delay determination according to the prior art is that the foregoing peak determination assumes that the ranging subchannel utilizes consecutive subcarriers (i.e., subcarriers which are adjacent to one another, or nearest one another with some guard band disposed between, within the spectrum of available bandwidth) and/or which are uniformly distributed (i.e., consistent spacing is provided between all subcarriers and their nearest neighbor subcarriers). Where such ranging subchannel subcarriers are consecutive and/or uniformly spaced, the foregoing IFFT calculation results in several relatively clearly defined peaks corresponding to different ranging signal paths. However, it has been discovered that the ranging subchannel subcarriers are often not consecutive and/or uniformly spaced. Where the ranging subchannel subcarriers are not consecutive and/or not uniformly spaced, the foregoing IFFT calculation contains many sidelobes (spurious peaks), further obscuring the peak associated with the ranging signal first path. Accordingly, the traditional techniques for determining time delay have not been capable of reliably establishing the timing of the first arriving path.